Beyond the actual features, there are some reasons such as the percent of the browser market, the company behind the browser, etc. These reasons are really subjective though, some may want a leading browser (for computability), some may want a less known one (for security), some may want a browser that is open source (like Firefox).

IE8 is my default on click simply due to many sites not working right elsewhere. Other than that I try not to use it since it crashes so often. Sites that I know work in other browsers, I will open with Chrome which is fast and stable.

Safari is getting close to Chrome, but since it doesn't do anything better that I care about, I don't use it. Opera is nice, but once again, doesn't offer anything better, and their political posturing bugs me. Firefox is slow and has strange printing problems on some of my systems, but if I need some random weird plug-in, I'll install it again.

Usability. I don't need add-ons, fancy cookie Management or skins, but I do need stuff like a working URL bar (Which is why I switched from the horrible Firefox 3.0 to Chrome, a browser that has it's fair share of problems but is at least usable for browsing.)

For me as a web developer the FireBug debugging tool is a reason to chose a Firefox as most used browser.

Some weird limitations in corporate tools (that works well only in Internet Explorer 6-7) is the reason while Internet Explorer 7.0 is my default browser.

And anyway I need to use almost all other browsers with market share > 1% from time to time to understand differences, strengths and weaknesses of each and better understand users of our web sites. Each of the browsers - Internet Explorer, Firefox, Google Chrome and Opera has its own advantages and bugs... So there is no killer browser nowadays, and the UI is just a matter of taste.

For the sake of users, developers, please also use IE version 8.0 or at least test your stuff here! :) Some web sites aren't working in IE80 even in compatibility mode :(
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Bogdan_ChJul 27 '09 at 17:31

@Bogdan_Ch - that is scary... IE8 compatibility mode should be almost exactly like IE7... Are there many sites you know of that don't work in compatibility mode?
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scunliffeJul 27 '09 at 20:37

I really like when the browser start respecting your habituation, lets you create new good habits and facilitate your browsing.

Chrome for instance, puts the address bar and the web search in a single place. When I find the web page I want I type tab and it gets autocompleted ( It respects my habit of pressing tab to auto complete.)

Firefox does something similar, it lets me go very fast to an already typed site by remembering them all. The only problem I had with it is I have to press down to get the result.

Safari on the other hand is very very fast, but doesn't quite match my previous habits.

Back in 1999, I was annoyed by some websites spamming me with JavaScript generated new windows, which generated 2 more windows when you hit the close button
in Internet Explorer 5. I looked for an alternative that gave me more control over JavaScript, and found Opera.

I suddenly had tabbed browsing, the ability to toggle JavaScript on or off with a single hotkey and a rendering engine that started displaying things before the page was loaded completely (1999, the previous millenium, I was still surfing with a 56k modem).

So my reason for choosing this browser was, that I was strongly annoyed by some behaviour that was not caused by my old browser. I still use my choice from that time today (right now, in fact) because I have no incentive to switch: I am not annoyed by my current browser, and I see no advantage in any of the alternatives that makes switching worth the effort. I guess you can call that lazyness...

One of the main things I take into account is if the browser controls the resources that can be spawned by each page. Some browsers are obnoxious about consuming all available CPU for a time while it decides what to do with a page.

I have found that Firefox is pretty good at not letting this happen which is why I use that instead of Internet Explorer. Nothing like having your computer lock up when you have a quad-core and 4 GB of memory.

I usually tend to stick with one browser until it gets on my nerves too often to be usable for me. Also trying out something new I have tried before in an older version is sometimes an incentive (to look whether my previous issues have been addressed).

I left Internet Explorer 7 because of speed in favor of Firefox, I left Firefox again due to excessive crashing when opening multiple tabs in quick succession (something I did fairly often at that time .. and a handful of crashes a day wasn't fun anymore). I started using Internet Explorer 8 β1 at that time but soon left it to try Firefox 3 and then Google Chrome, due to speed and other issues. Chrome, while snappy and overall nice constantly irritated me when I clicked on a tab and rather often the tab was detached from the window, resulting in a nasty pause. By now I'm a fairly happy user of Internet Explorer 8 which is nearly as fast as Chrome and (currently) has less issues for me. I still use Chrome and Firefox 3 on my other machine, though.